1. Field of the Invention
The present invention relates to laser diode arrays, and more specifically, it relates to a low-cost laser diode array having laser diodes with precisely registered positions within a monolithic submount.
2. Description of Related Art
Laser diode arrays are used in a wide range of commercial, medical and military applications: materials processing (soldering, cutting, metal hardening), display technology/graphics, medical imaging and surgical procedures (corneal shaping, tissue fusion, dermatology, photodynamic therapy), satellite communication, remote sensing, and laser isotope separation. In certain solid-state laser applications it is desirable to use laser diode arrays to optically excite, ie., "pump," the crystal hosts. Diodes offer a narrow band of emission (reducing thermal lensing), compactness, high electrical efficiency and higher reliability as compared to flash lamps. Despite these numerous advantages, however, diode-pumped solid-state lasers (DPSSLs) have gained slow market acceptance due to the high cost associated with the laser diode array pumps. Significant diode array cost reductions would enable wide deployment of DPSSLs and new architectures to be realized that were previously cost prohibitive. In particular, low-cost diode arrays would bolster the inertial confinement fusion (ICF) and inertial fusion energy (IFE) programs that require low-repetition rate laser diode arrays in very high volumes.
Historically, much of the research and development in this area was devoted to solving diode material and fabrication issues in order to improve the yield and reliability of laser diodes. High quality InAlGaAs and InGaAsP laser diodes are now commercially available for pumping Nd:YAG at .about.810 nm. As much as 100 W/cm of peak power is possible under pulsed operation, and over 10,000 hours of continuous operation (CW) in commercial systems has been demonstrated at reduced power levels (20-30 W CW). Although these types of performance improvements have led to cost reductions in the past, there has not been a complementary improvement in the packaging technology, which is now limiting further cost reductions from being achieved.
Most packaging/heatsink schemes use a "rack and stack" architecture. In this method, individual laser bars are fabricated into sub-assemblies, and the sub-assemblies are then bonded together to produce larger two-dimensional arrays. Labor intensive steps associated with handling individual components prevents the production of arrays in large volume and in high yield. To alleviate this problem, a "monolithic" fabrication approach known as "bars-in-grooves" was proposed. This process was commercialized by Laser Diode Array Inc. and it represents the only "monolithic" technology that is commercially available today. There are trade offs associated with using a monolithic technique (e.g. by LDA Inc.) and the salient issues are discussed below.
The grooves must be deliberately over-sized to facilitate mounting the bars (as well as to allow for a range of diode bar thicknesses). Accurate final placement of the laser bar is therefore difficult to achieve as solder is used to fill in the void left by the over-sized grooves. This prohibits accurate collimation (lensing) of the laser diodes which is desirable in "high-brightness" applications that are often used in "end-pumped" geometries. More importantly, flowing solder around the bars can damage, or short-out bars which lowers yield and represents a serious liability to packaging costs of a completed array. Either that, or the strict process controls and/or lower yield of "suitable" bars that is necessary poses a cost penalty with this soldering technique. The following invention improves upon the limitations of the former "bars-in-grooves" method, while still benefiting from being a monolithic or quasi-monolithic approach. The placement of the laser diodes is well defined, and the soldering process can be extremely well controlled, or not used at all, which ensures a high yield that is crucial for a low-cost high yield production of laser diode arrays.
U.S. Pat. No. 5,394,426 is directed to a low-cost diode laser bar assembly that increases peak power output and improves heat transfer and removal by using a thin, electrically insulating polyimide film-underneath the diode laser bar array. The exposed portions of the insulating film are metalized to allow indium solder bonding of the laser bar assembly to a heat sink. An interconnecting layer (between the metalization layer and the substrate) secures the diode laser bar assembly to the heat sink. This patent is not directed to a monolithic diode laser array that uses an electrically-conductive ribbon and sidewalls metalized on only one-side.
U.S. Pat. No. 5,284,790 and U.S. Pat. No. 5,040,187 are directed to the method of fabricating and the apparatus for a low-cost monolithic laser diode array, respectively. One or more grooves are formed on a monolithic substrate and the sidewall of the groove(s) is metalized. Laser diodes can be placed into the grooves by using a flexible monolithic substrate that can be bent just long enough to insert the laser diode. The force of the metalized walls of the groove holds the laser diode bar in place. Alternatively, a larger metalized groove can be made, and the laser diode can be inserted and held in place by some type of bonding/adhesive substance. These patents are not directed to a monolithic diode laser that uses an electrically-conductive ribbon U.S. Pat. No. 5,040,187 describes a method for flexing the substrate in order to facilitate loading laser bars. However, both the groove width and diode bar thickness must be controlled to such a high level of accuracy that this approach appears to be impractical. The present invention allows a practical implementation because individual elastomers can accommodate any thickness variations of the individual components.
U.S. Pat. No. 5,357,536 is directed to the method and the apparatus for positioning laser diodes. Parallel structures, at preset intervals, are made on the surface of the heatsink. The diode lasers, which have raised waveguides attached to their surfaces, are placed on the heatsink such that the waveguides are abutting the parallel structures. The laser diodes can then be permanently attached to the heatsink. This patent is not directed to a monolithic diode laser array that uses metalized sidewalls and an electrically-conductive ribbon.
U.S. Pat. No. 5,128,951 is directed to the method and the apparatus for a laser diode array that does not require separate metalization of the grooves in which the diode lasers are placed. One method is to bond/grow/plate a conductive layer to an electrically insulating or semi-insulating layer. Grooves are then cut or etched into the conductive layer. Alternatively, the upper portion of a substrate may be doped to make the substrate highly conductive. The undoped, lower portions of the substrate are then nonconductive. This patent is not directed to a monolithic diode laser array that uses an electrically-conductive ribbon and is metalized by a single, angled evaporation.
U.S. Pat. No. 4,881,237 is directed to hybrid two-dimensional surface-emitting laser arrays. Edge-emitting lasers are placed into grooves in a highly heat-conductive substrate (Si or Cu). The grooves are made such that the sidewalls are at 45.degree. angles. Microchannels can be made in the substrate to hold a cooling fluid. This patent is not directed to a monolithic diode laser array that uses an electrically-conductive ribbon and sidewalls metalized on only one-side.